An unclassified member of the bacterial family Pasteurellaceae has been isolated from cases of enteritis, conjunctivitis, and abscessation in a colony of golden hamsters (Mesocricetus auratus). This bacterium is a member of the Haemophilus/Pasteurella group, based on routine biochemical testing. It may represent a new species, but additional studies such as DNA hybridization may help to define the taxonomic position. Three golden hamster strains are maintained in the NIH Animal Genetic Resource - N:Syr (Syrian), N:M70 (Murphy), and N:LSH/TrN. The Haemophilus/Pasteurella group bacterium has been isolated from all strains. Both clinically ill and healthy adult hamsters representing all strains have been examined for gross and microscopic abnormalities, identification of gastrointestinal parasites, characterization of intestinal microflora, and identification of pathogenic bacteria at sites of inflammation. Syrian hamsters from outside the colony have been used to determine the relative pathogenicity of the Haemophilus/Pasteurella group bacterium. Various routes of inoculation and doses of bacteria have been combined with clinical observations and post-mortem studies. This unique bacterial species in the Haemophilus/Pasteurella group is widespread in the hamster colony and may be responsible for significant morbidity and mortality in this colony. This study will help define the lesions produced, extent of the disease in the colony, and will result in the development of appropriate techniques to eliminate this infection from the colony and thus improve the health of animals used by NIH scientists.